The present invention relates generally to fabrication of interconnect structures within integrated circuits, and more particularly, to minimizing electromigration failure and degradation of an interconnect structure formed in porous low-K dielectric material.
A long-recognized important objective in the constant advancement of monolithic IC (Integrated Circuit) technology is the scaling-down of IC dimensions. Such scaling-down of IC dimensions reduces area capacitance and is critical to obtaining higher speed performance of integrated circuits. Moreover, reducing the area of an IC die leads to higher yield in IC fabrication. Such advantages are a driving force to constantly scale down IC dimensions.
Thus far, aluminum has been prevalently used for metallization within integrated circuits. However, as the width of metal lines are scaled down to smaller submicron and even nanometer dimensions, aluminum metallization shows electromigration failure. Electromigration failure, which may lead to open and extruded metal lines, is now a commonly recognized problem. Moreover, as dimensions of metal lines further decrease, metal line resistance increases substantially, and this increase in line resistance may adversely affect circuit performance.
Given the concerns of electromigration and line resistance with smaller metal lines and vias, copper is considered a more viable metal for smaller metallization dimensions. Copper has lower bulk resistivity and potentially higher electromigration tolerance than aluminum. Both the lower bulk resistivity and the higher electromigration tolerance improve circuit performance.
Referring to FIG. 1, a cross sectional view is shown of a copper interconnect 102 within a trench 104 formed in an insulating layer 106. The copper interconnect 102 within the insulating layer 106 is formed on a dense dielectric material 109 deposited on a semiconductor wafer 108 such as a silicon substrate as part of an integrated circuit. The dense dielectric material 109 may be a hardmask layer, an etch stop layer, or a capping layer comprised of SiO2 (silicon dioxide) or SiN (silicon nitride) for example. Because copper is not a volatile metal, copper cannot be easily etched away in a deposition and etching process as typically used for aluminum metallization. Thus, the copper interconnect 102 is typically formed by etching the trench 104 as an opening within the insulating layer 106, and the trench 104 is then filled with copper typically by an electroplating process, as known to one of ordinary skill in the art of integrated circuit fabrication.
Unfortunately, copper is a mid-bandgap impurity in silicon and silicon dioxide. Thus, copper may diffuse easily into these common integrated circuit materials. Referring to FIG. 1, the insulating layer 106 may be comprised of silicon dioxide or a low dielectric constant insulating material such as organic doped silica, as known to one of ordinary skill in the art of integrated circuit fabrication. The low dielectric constant insulating material has a dielectric constant that is lower than that of pure silicon dioxide (SiO2) for lower capacitance of the interconnect, as known to one of ordinary skill in the art of integrated circuit fabrication.
Copper may easily diffuse into such an insulating layer 106, and this diffusion of copper may degrade the performance of the integrated circuit. Thus, a diffusion barrier material 110 is deposited to surround the copper interconnect 102 within the insulating layer 106 on the sidewalls and the bottom wall of the copper interconnect 102, as known to one of ordinary skill in the art of integrated circuit fabrication. The diffusion barrier material 110 is disposed between the copper interconnect 102 and the insulating layer 106 for preventing diffusion of copper from the copper interconnect 102 to the insulating layer 106 to preserve the integrity of the insulating layer 106.
Further referring to FIG. 1, an encapsulating layer 112 is deposited as a passivation layer to encapsulate the copper interconnect 102, as known to one of ordinary skill in the art of integrated circuit fabrication. The encapsulating layer 112 is typically comprised of a dielectric such as silicon nitride, and copper from the copper interconnect 102 does not easily diffuse into such a dielectric of the encapsulating layer 112.
As the density of integrated circuit structures continually increases, the distance between the interconnect structures decreases. As the distance between the interconnect structures decreases, a dielectric material with a low dielectric constant (i.e., a low-K dielectric material) is desired for the insulating layer 106. The insulating layer 106 being comprised of the dielectric material with a low dielectric constant results in lower capacitance between the interconnect structures. Such lower capacitance results in higher speed performance of the integrated circuit and also in lower power dissipation. In addition, such lower capacitance results in lower cross-talk between the interconnect structures. Lower cross-talk between interconnect structures is especially advantageous when the interconnect structures are disposed closer together as device density continually increases.
Referring to FIG. 2, one example of a dielectric material with a low dielectric constant for the insulating layer 106 is a porous dielectric material having pores throughout as known to one of ordinary skill in the art of integrated circuit fabrication. An interconnect opening 116 is formed within the porous dielectric material of the insulating layer 106 for forming an interconnect structure within the interconnect opening 116. With the porous dielectric material for the insulating layer, the interconnect opening 106 has opened pores 118 at the sidewalls of the interconnect opening 116.
Referring to FIG. 3, in the prior art, as device dimensions continually decrease, a diffusion barrier material 120 that is as thin as possible is deposited on the sidewalls and the bottom wall of the interconnect opening 116. The diffusion barrier material 120 of FIG. 3 is similar to the diffusion barrier material 110 of FIG. 1. Because diffusion barrier materials generally have higher resistance than copper, the diffusion barrier material 120 in the prior art is deposited as thin as possible to minimize resistance of the interconnect structure formed within the interconnect opening 116.
However, the pores 114 of the insulating layer 106 range in size with the pores 114 having a mean diameter in a range of from about 10 xc3x85 (angstroms) to about 200 xc3x85 (angstroms), depending on the type of low-K dielectric material of the insulating layer 106. For example, for OSG (organic spin-on glass), the diameter of the pores are in a range of from about 10 xc3x85 (angstroms) to about 40 xc3x85 (angstroms) with the mean diameter being about 25 xc3x85 (angstroms). On the other hand, for hydrocarbon polymer material, the diameter of the pores are in a higher range being as high as 200 xc3x85 (angstroms).
A thin diffusion barrier material 120 that is deposited in a conformal deposition process of the prior art does not completely fill such opened pores 118 at the sidewalls of the interconnect opening 116. Further referring to FIG. 3, when the diffusion barrier material 120 does not completely fill such opened pores 118 at the sidewalls of the interconnect opening 116, a seed layer of copper 122 that is deposited onto the diffusion barrier material 120 does not reach into the opened pores 118 at the sidewalls of the interconnect opening 116.
Referring to FIG. 4, when a copper conductive fill 124 is grown from the copper seed layer 122 for filling the interconnect opening 116, voids 126 are formed from the opened pores 118 at the sidewalls of the interconnect opening 116 because the opened pores 118 do not have the copper seed layer 122 deposited therein. Such voids 126 disadvantageously increase the probability of electromigration failure of the interconnect structure.
Nevertheless, use of porous dielectric material for the insulating layer 106 is desired for the low dielectric constant. Accordingly, a mechanism is desired for preventing formation of voids from opened pores at the sidewalls of the interconnect opening formed within the porous dielectric material.
In addition, one type of porous low-K dielectric material is formed from a low-K precursor material comprised of a thermosetting host material and a thermally degradable xe2x80x9cporogenxe2x80x9d material, as known to one of ordinary skill in the art of integrated circuit fabrication. In that case, a solution of the low-K precursor material is applied by a spin-on process, and then a thermal process is performed for xe2x80x9ccuringxe2x80x9d the low-K precursor material to the porous low-K dielectric material. The thermal process causes curing of the low-K precursor material with cross-linking of the host material to form a low-K dielectric matrix and concurrently with phase separation of the porogen from the host material. The phase separated porogen collects in nanoscopic domains within the host material and thermally decomposes into volatile by-products (i.e., porogen fragments) that diffuse out of the low-K dielectric material leaving pores in their place. Dow Chemical""s porous SILK product and JSR Corporation""s JSR 5109 product are examples of commercially available precursors that utilize an organic host material. Such formation of low-K porous dielectric material by curing low-K precursor material in a thermal curing process is known to one of ordinary skill in the art of integrated circuit fabrication.
However, the porogen fragments that diffuse out during the thermal process of curing the low-K dielectric material may contain carbon, oxygen, chlorine, and/or hydrogen and compounds of such elements that degrade copper interconnect upon contact with the copper interconnect. Thus, a mechanism is desired for preventing contact of the porogen fragments with a surface of copper interconnect structure.
Accordingly, in a general aspect of the present invention, an interconnect opening is formed in a low-K precursor material that is not completely cured such that no significant pores are opened at sidewalls of the interconnect opening when filling the interconnect opening with conductive fill material. In addition, a capping material comprised of an amorphous alloy or a microcrystalline alloy having stuffed grain boundaries is formed on the conductive fill material before a thermal curing process for curing the low-K precursor dielectric material to become the porous low-K dielectric material.
In a general aspect of the present invention, for fabricating an interconnect structure within an interconnect opening formed within a porous dielectric material, the interconnect opening is initially formed within a low-K precursor material that is not completely cured. The interconnect opening is then filled with a conductive fill material being contained within the interconnect opening and with a top surface of the conductive fill material within the interconnect opening being exposed. A capping material is formed on the top surface of the conductive fill material, and the capping material is an amorphous alloy or is a microcrystalline alloy having stuffed grain boundaries. A thermal curing process is then performed for curing the low-K precursor material to become a porous low-K dielectric material. The capping material on the top surface of the conductive fill material is impervious to at least one of oxygen, carbon, hydrogen, chlorine, and porogen fragments that are generated as out-gassing volatile by-products from the low-K precursor material during the thermal curing process.
In another embodiment of the present invention, a diffusion barrier material is also formed at the sidewalls of the interconnect opening between the conductive fill material and the low-K precursor material that is not completely cured. In that case, the capping material is selectively formed on any exposed surface of the conductive fill material and the diffusion barrier material before the thermal curing process.
In a further embodiment of the present invention, an activation layer is formed before the capping material is formed on the activation layer for enhancing adhesion of the capping material to the conductive fill material.
The present invention may be used to particular advantage for forming an interconnect structure within the interconnect opening that is one of a metal line, a via hole, or a dual damascene opening.
In this manner, because the interconnect opening is formed and filled within the low-K precursor material that is not completely cured, the sidewalls of such an interconnect opening do not have any significantly sized opened pores during filling of the interconnect opening. Thus, formation of voids, and in turn, electromigration failure of the interconnect structure is minimized. In addition, the capping material that is an amorphous alloy or a microcrystalline alloy having stuffed grain boundaries is impervious to at least one of oxygen, carbon, hydrogen, chlorine, and porogen fragments that are generated as out-gassing volatile by-products from the low-K precursor material during the thermal curing process. Since such a capping material encapsulates the top surface of the conductive fill material, such volatile by-products of the low-K precursor material do not contact the conductive fill material to preserve the integrity of the conductive fill material during curing of the low-K precursor material to form the porous low-K dielectric material.
In another aspect of the present invention, for fabricating an interconnect structure, an interconnect opening is formed within a porous dielectric material with opened pores at sidewalls of the interconnect opening. A diffusion barrier material is formed at a bottom wall of the interconnect opening. The diffusion barrier material is then sputtered away from the bottom wall of the interconnect opening and onto the sidewalls of the interconnect opening to substantially fill the opened pores at the sidewalls with the diffusion barrier material. The interconnect opening is then filled with a conductive fill material after the opened pores at the sidewalls of the interconnect opening are filled with the diffusion barrier material.
In one embodiment of the present invention, the diffusion barrier material is deposited onto the bottom wall of the interconnect opening in an electroless deposition process after the interconnect opening is formed. In another embodiment of the present invention, the diffusion barrier material is formed onto a conductive material of a bottom interconnect structure, and then the porous dielectric material and the interconnect opening are formed over the diffusion barrier material such that the diffusion barrier material forms the bottom wall of the interconnect opening.
In this manner, because the interconnect opening is filled with the conductive fill material after the opened pores at the sidewalls of the interconnect opening are filled with the diffusion barrier material, void formation from the opened pores are minimized to in turn minimize electromigration failure of the interconnect structure.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.